The present invention relates to a tire sensor, especially tire sidewall torsion sensor=SWT sensor, including at least two pick-ups for measuring data being mounted at a distance from the tire rotational axis on the chassis, the said pick-ups for measuring data interacting with at least one encoder mounted on or in the tire wall or with at least one conventional encoder mounted on or in the tire wall and exhibiting poles, wherein the output signals or output information of such pick-ups for measuring data sensors are/is transmitted to the motor vehicle control system.
Many methods and devices using tire sensors for detecting the forces and torque acting on the tires are known for controlling the driving performance of a motor vehicle. Tire sensors (SWT sensors) consist of one encoder mounted in or on the tire and at least one pick-up for measuring data that is mounted on the chassis in a stationary manner and associated with the encoder. Whereas EP 04 441 09 B1 proposes monitoring the deformation of the range of the tire profile, i.e., the tire contact area, WO 96/10505 proposes detecting the deformation of the sidewall of a tirexe2x80x94the torsion deformationxe2x80x94by measuring a period of time that elapses between the passing of at least two markings arranged on the rotating wheel at a different radius in relation to the rotational axis. The longitudinal forces that act on the tire are inferred from the evaluation of the signals obtained as described above. In addition, the transverse forces may be determined from variations of the amplitudes of the signal sensed by the pick-up for measuring data which represents changes in the distance between the pick-up for measuring data and the encoder. A tire sensor which detects a change in the phase position between output signals emitted by pick-ups for measuring data when the tire is deformed due to forces acting on the tire is described in WO 97/44673.
Further, a method is disclosed in WO 99/19192 which, on the basis of the forces that act on the individual wheels and are sensed by tire sensors, determines condition variables of the vehicle which satisfy the high demands placed on motor vehicle control systems in terms of accuracy and reliability.
An object of the present invention is to send the signals provided by the tire sensor to an evaluation unit or the motor vehicle control system rid of errors that are due to influence quantities.
According to the present invention, this object is achieved by a generic tire sensor with the special characteristics that at least one analog and at least one digital signal conditioning and/or processing unit are provided between the motor vehicle control system and the pick-ups for measuring data, that the pick-ups for measuring data send at least two output signals that can be evaluated with respect to a change in the phase position and/or at least one output signal that can be evaluated with respect to the change of the amplitude to the digital signal-conditioning and/processing unit, and that the digital signal-conditioning and/or processing unit calibrates the systematic errors of the output signals with respect to whether the error is an amplitude-related error or a phase-related error.
The tire sensor of the present invention and a control based thereon, hence, founds on the forces that occur directly at the tire. This permits detecting all influence quantities and wrong interpretations which are due to ambiguous signals or errors in processing what impairs the determination of vehicle condition variables or quantities describing the vehicle behavior. An error quantity which is the basis of the respective electric characteristic quantities and is caused by different influence quantities is taken into consideration by the calibration of the generally sinusoidal signals, provided by the pick-up(s) for measuring data with respect to the signals"" electric characteristic quantity. The envisaged signal-conditioning and/or processing unit reduces the quantity of data of several interlinked analog input signals in a favorable manner so that further processing of the information obtained is ensured with reduced effort and structure in the digital signal processing. In this arrangement, the changes of the amplitude, the period duration, and the phase relation of the input signals permits obtaining the information which render it possible to calculate the transverse and longitudinal forces that act on the tires. The signal-conditioning and/or processing unit comprises a means to detect undesirable error quantities and to compensate them numerically in the subsequent data processing operation.
In a favorable embodiment of the present invention, the amplitude-related error is compensated by a multiplicative calibration and the phase-related error is compensated by an additive calibration in the digital signal-conditioning and/or processing unit. According to the present invention, the amplitude-related output signal is rectified and the maximum values (amplitudes) determined in the analog signal-conditioning unit, while in the digital signal-conditioning and/or processing unit the offset is determined by way of averaging the maximum values with correct signs and a multiplier f is formed of the maximum value divided by the mean value of the rectified signal and assigned to each output signal or each pole of the encoder. Associated with the phase-related output signal of the tire sensor is, according to the present invention, an ideal pole pattern which corresponds to the number of poles and preferably has equidistant poles. In the digital signal-conditioning and/or processing unit, the ideal pole pattern is compared to each pole of the encoder of the phase-related output signal. In dependence on the comparison, correction factors are then formed or updated which are associated additively with the output signal according to the present invention.
Thus, the present invention is based on the knowledge that the correction of the amplitude error can be based on; the assumption that the error is a multiplicative error because additive air slot variations (error: xcex94) or tire sidewall equalities can be presupposed which, by way of the exponential characteristic curve
|Amplitudexe2x88x92Offset|=A*exp(B*air slot) become a multiplicative amplitude variation:
|Amplitudexe2x88x92Offset|=A*exp(B*(air slot+xcex94)
|Amplitudexe2x88x92Offset|=A*exp(B*air slot+xcex94xe2x80x2)
|Amplitudexe2x88x92Offset|=A*exp(B*air slot)* exp(xcex94xe2x80x2)
|Amplitudexe2x88x92Offset|=A*exp(B*air slot)* xcex94xe2x80x3
and a multiplicative inequality of the magnetic field is presupposed which is directly proportional to the amplitude and thereby generates a multiplicative inequality of the amplitude.
The correction of the pole division error, however, is based on the assumption that the pole division error is an additive error because it is an angle error which changes additively over the periphery of the encoder 17.
Favorably, the amplitudes and phase differences are stored in a ring memory for error correction, corresponding to the pole number, such as 1, 2, . . . to 96.
The calibration of the present invention permits immediately correcting errors that occur periodically on a wheel, without the need for effecting a time-consuming (delay time) filtering operation. It was found out in tests that the accuracy of signal evaluation can be enhanced by up to 7 percent due to the calibration.